Polytetrafluoroethylene is well known for its ability to withstand the environmental conditions to which oil seals are exposed. However, its use has been limited by its inability to be molded in an efficient manner. This problem has been solved heretofore by first grooving the end of a billet of polytetrafluoroethylene and thereafter cutting the polytetrafluoroethylene seal element from the billet as taught in, for example, applicant's copending application Ser. No. 322,640 for Bi-Directional Hydrodynamic Seal, as well as U.S. Pat. Nos. 3,857,156; 2,606,779 and 4,132,421.
Patents that teach the use of built-up ribs to effect hydrodynamic pumping of oil are U.S. Pat. Nos. 4,118,856 and 3,620,540. A teaching of both ribs and grooves is found in the "Journal of Teflon", pages 2-4, January 1970. However both open grooves and built-up ribs exhibit manufacturing and operational problems to which the present invention presents a solution.
As additional background to the present invention, it is to be noted that a fluid is a substance that deforms when subjected to shear stress, no matter how small that shear stress may be. Shear stress is the stress component tangent to a surface. The average shear stress over an area is the stress divided by the area of the surface.
When a fluid is placed between two closely spaced smooth parallel plates and a force F is applied to one plate, for example, a shaft journaled in a seal, a shear stress F/A, where A is the area of the shaft, is exerted on oil disposed between the plates, or in the example of this invention on oil between the shaft and the shaft seal. Each fluid particle moves parallel to the plates and its velocity varies uniformly from zero at the stationary plate or seal to U at the upper plate or shaft, and F is directly proportional to A and to U and inversely proportional to thickness t. In equation form ##EQU1## in which u is a proportionality factor relating to a particular fluid. If .tau.=F/A for the shear stress, ##EQU2## The ratio U/t is the rate of angular deformation of the fluid. The angular velocity may also be written du/dy, as both U/t and du/dy express the velocity change divided by the distance over which the change occurs. The velocity gradient du/dy may also be visualized as the rate at which one layer moves relative to an adjacent layer. In differential form, ##EQU3## which is known as Newton's law of viscosity, is the relation between shear stress and rate of angular deformation for one-dimensional flow of a fluid. The proportionality factor .mu. is called the viscosity of the fluid.